The invention relates to a method for the manufacture of tubes bent in a U-shape from a nonferrous metal, immediately following a tube production line. The invention also relates to a production line for the manufacture of tubes bent in a U-shape.
Cold air has to be provided for the air-conditioning of buildings and the generation of low temperatures in the industrial sector (for example, cold-storage rooms, warehouses). Cyclic processes are generally used for this purpose. A distinction is made between compression processes and absorption processes. Lamellar heat exchangers in which air is cooled by a cold heat carrier or by a directly evaporating refrigerant are used to generate cold air. In air-cooled refrigerating/air-conditioning systems, the waste heat occurring in a condenser or a re-cooling plant is conveyed away to the atmospheric air. Both types of heat exchangers are typically in the form of lamellar apparatuses which comprise stacks of lamellae fitted on copper tubes.
Depending on the thermo-hydraulic requirements, the lamellar apparatuses are constructed with one or more tube passes. Whereas straight tubes are generally used in apparatuses having one pass, the refrigerant stream or heat carrier stream has to be reversed in multi-pass constructions and conveyed back through the apparatus. These reversals of direction can be effected by fittings (bends) attached to the tube ends or by tubes bent in a U-shape. This last-mentioned solution is very inexpensive and therefore the industrially advantageous solution.
In order to manufacture the lamella stacks, the thin-walled tubes, which are generally obtained in coiled form and which have a wall thickness of only a few tenths of a millimeter, are uncoiled by the heat exchanger manufacturer, cut to a desired length and bent to form U-shaped tubes (“hairpins”).
Bending machines for U-shaped tubes permit the simultaneous bending of up to eight hairpins (or more) at the same time. However, in the case of multiple-bending machines, the entire bending line comes to a standstill when a coil runs out. The exhausted coil must then be replaced by a fresh one. Large coil weights lead to fewer stops but are substantially more demanding in terms of handling during transport and coil changes. Many uncoiling devices are not designed for relatively large coil weights. In order to avoid frequent coil changes, apparatus manufacturers stipulate minimum coil weights. Underweight coils, which occur at the tube manufacturer's as a result of the manufacturing process, are often not saleable and have to be scrapped.
During the uncoiling process the tubes are alternately accelerated and braked. During that operation, the thin-walled tubes are susceptible to buckling. In the case of uncoiling devices that are not braked, loops are sometimes formed which may cause the coiled layers to become “unraveled”. After a few rotations, this may result in the overlapping of the layers with snarling and finally in the tube breaking.
In addition, the manufacture of tubes from copper and copper alloys for the areas of application to be considered here, as domestic plumbing tubes and especially as heat exchanger tubes for air-conditioning and refrigeration engineering, is known.
For example, starting from cylindrical slabs or billets cast by the ingot-casting method or by the continuous-casting method, the extrusion of tubes and the hot-rolling of tube blanks are applied as the currently normal method of hot-forming for large reductions in cross-section. In the subsequent production step, cold-forming methods, such as slide drawing with slide drawing machines or ball block machines, or tube-rolling techniques, such as cold pilger rolling, are used.
With the planetary diagonal rolling process, a reduction in cross-section of the tube wall of more than 90% is achieved in one pass by means of a continuously running tube-rolling process. The resultant frictional heat in the forming region leads to a heating of the material up to the recrystallisation range of copper. Such a temperature treatment has a direct effect on the softness of the material in order to ensure adequate deformability for the subsequent cold-drawing steps. In order to at least partially suppress the enormous increase in temperature, printed specification EP 1 232 808 A2 proposes a cooling device for the cold-rolling operation.
Printed specification EP 0 648 855 A1 describes, for example, a method for the manufacture of seamless tubes from copper and copper alloys, in which method a special role is played by targeted recrystallisation processes in relation to deformability. The method also describes the usual handling of semi-finished tubes which are wound into a coil after a forming process. A feature of present-day practice in the production of heat exchanger tubes from copper and copper alloys is that the final form is wound predominantly into multi-turn coils having narrow coil radii and a limited weight and is delivered in the bright-annealed soft state in order to be unwound again at another site, straightened and cut to length and further processed to form, for example, tubes bent in the shape of a hairpin for lamellar heat exchangers.
This hitherto conventional net product chain from tube manufacture through the production of heat exchangers to the installation of complete refrigerating/air-conditioning systems, has grown historically. It is based on the fact that tubes are common semi-finished products which are manufactured by the semi-finishing industry and delivered to the apparatus manufacturers. Since tube manufacturers and apparatus constructors are often far apart geographically, the shipment of coiled tubes, which require only a small amount of transport volume, has developed.
It is precisely the tight winding of the tube coils which creates fundamental problems in the handling of the material. In the case of conventional dimensions of the wound coils with a diameter of 0.6 m and a maximum outside coil diameter of approximately double that figure, the tubes, after the bend-straightening operations which have taken place on removal from the basket and conveyance into the coiler, with expansions and compressions of 1-3%, which are a function of the outside tube diameter, are subjected to fresh forming under bending conditions. The bend-straightening, carried out under longitudinal tensile stresses, to return to a straight tube again, result in a reduction in its average outside diameter, and the deflection onto the winder results in a reduction in wall thickness in the outer expansion region and to an increase in wall thickness in the inner compression region and in a flattening and ovalisation of its cross-section. Because the coil weights constitute, in a limited manner, only approximately one quarter to a maximum of approximately half the total weight of the tube conductor length deposited in the basket, considerable tube waste often also occurs.
In addition, the softening and recrystallisation which occur during the bright-annealing of the highly hardened heat exchanger tubes, which are in the form of tightly wound coils, lead to an adaptation of the tube cross-sections to the geometrical constrained conditions of the coil and therefore to changes in the shape of the tube cross-section and in the layer diameters of individual turns, especially the outer turns.
Further disadvantages of the hairpin production of tightly coiled and soft tubes are surface damage, jamming turns, which result in buckling, and the occurrence of a large amount of scrap owing to unusable tube ends at the beginning and the end of the coil.